Mutual antagonism

Figure 8.9c, derived from the Lotka-Volterra model, describes a situation in which interspecific competition is, for both species, a more powerful force than intraspecific competition. This is known as mutual antagonism.

An extreme example of such a situation is provided by work on two species of flour beetle: Tribolium con-fusum and T. castaneum (Park, 1962). Park's experiments in the 1940s, the Competitive Exclusion Principle difficulty proving and, especially, disproving the Principle niche differentiation and interspecific competition: a pattern and a process not always linked reciprocal predation in flour beetles

Table 8.1 Reciprocal prédation (a form of mutual antagonism) between two species of flour beetle, Tribolium confusum and T. castaneum. Both adults and larvae eat both eggs and pupae. In each case, and overall, the preference of each species for its own or the other species is indicated. Interspecific predation is more marked than intraspecific predation. (After Park et al., 1965.)

'Predator' 'Shows a preference for...'

Table 8.1 Reciprocal prédation (a form of mutual antagonism) between two species of flour beetle, Tribolium confusum and T. castaneum. Both adults and larvae eat both eggs and pupae. In each case, and overall, the preference of each species for its own or the other species is indicated. Interspecific predation is more marked than intraspecific predation. (After Park et al., 1965.)

'Predator' 'Shows a preference for...'

Adults eating eggs

T.

confusum

T.

confusum

T.

castaneum

T.

confusum

Adults eating pupae

T.

confusum

T.

castaneum

T.

castaneum

T.

confusum

Larvae eating eggs

T.

confusum

T.

castaneum

T.

castaneum

T.

castaneum

Larvae eating pupae

T.

confusum

T.

castaneum

T.

castaneum

T.

confusum

Overall

T.

confusum

T.

castaneum

T.

castaneum

T.

confusum

1950s and 1960s were amongst the most influential in shaping ideas about interspecific competition. He reared the beetles in simple containers of flour, which provided fundamental and often realized niches for the eggs, larvae, pupae and adults of both species. There was certainly exploitation of common resources by the two species; but in addition, the beetles preyed upon each other. The larvae and adults ate eggs and pupae, cannibalizing their own species as well as attacking the other species, and their propensity for doing so is summarized in Table 8.1. The important point is that taken overall, beetles of both species ate more individuals of the other species than they did of their own. Thus, a crucial mechanism in the interaction of these competing species was reciprocal predation (i.e. mutual antagonism), and it is easy to see that both species were more affected by inter- than intraspecific predation.

Figure 8.9c, the Lotka-Volterra model, suggests that the consequences of mutual antagonism are essentially the same whatever the exact mechanism. Because species are affected more by inter- than intraspecific competition, the outcome is strongly dependent on the relative abundances of the competing species. The small amount of interspecific aggression displayed by a rare species will have relatively little effect on an abundant competitor; but the large amount of aggression displayed by an abundant species might easily drive a rare species to local extinction. Moreover, if abundances are finely balanced, a small change in relative abundance will be sufficient to shift the advantage from one species to the other. The outcome of competition will then be unpredictable - either species could exclude the other, depending on the exact densities that they start with or attain.

Table 8.2 Competition between Tribolium confusum and T. castaneum in a range of climates. One species is always eliminated and climate alters the outcome, but at intermediate climates the outcome is nevertheless probable rather than definite. (After Park, 1954.)

Percentage wins

Climate

T. confusum

T. castaneum

Hot-moist

0

100

Temperate-moist

14

86

Cold-moist

71

29

Hot-dry

90

10

Temperate-dry

87

13

Cold-dry

100

0

Table 8.2 shows that this was indeed the case with Park's flour beetles. There was always only one winner, and the balance between the species changed with climatic conditions. Yet at all intermediate climates the outcome was probable rather than definite. Even the inherently inferior competitor occasionally achieved a density at which it could outcompete the other species.

Table 8.2 shows that this was indeed the case with Park's flour beetles. There was always only one winner, and the balance between the species changed with climatic conditions. Yet at all intermediate climates the outcome was probable rather than definite. Even the inherently inferior competitor occasionally achieved a density at which it could outcompete the other species.

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